Species and speciation

Categories: Classification and systematics; ecology; evolution

Species are distinct kinds of organisms in that the organism can be recognized as different from other kinds of similar organisms by a combination of characteristic shapes, sizes, behaviors, physiology, or other attributes. For instance, white oaks can be recognized as different from other oak species by growth form and habitat, along with a combination of leaf, fruit, and bark characteristics. To be useful as a diagnostic feature, a characteristic must lend itself to measurement and remain relatively constant generation to generation. Such a hereditary pattern implies that members of a species share a common pool of genetic information.

Although most field guides and keys for species identification are based largely on measurable morphological traits, the biological species concept, attributed to Ernst Mayr in the 1940s, defines species as groups of interbreeding populations reproductively isolated from other such groups. Sometimes a greater range of variation can be observed in large, dispersed populations than is found between similar but reproductively isolated species. What, then, determines when a species is formed?

Unfortunately there is no clear-cut answer. Investigators may use different criteria or assign variable levels of importance to characteristics; therefore, a population of plants may be assigned species status by one expert and varietal status by another. While this is confusing to those searching for a name, the problem is indicative of the dynamic and changing nature of life. Because evolution is an ongoing process, some groups of plants are expected to be in transition.

The processes that contribute to variation in large, sexually reproducing populations also are responsible for the origin of species. Isolation and selection of genetically based variation are the only additional requirements. Speciation generally is conceived to involve the separation, isolation, and divergence of a genetic pool of information. Plants that share a common pool of genetic information are split into two pools that remain isolated until identifiable genetic differences accumulate. Classifying a segment of this pool as a species requires recognition of significant genetic differences and the relative isolation of the population.

Allopatric Speciation

The physical separation of a gene pool into populations that are geographically or spatially isolated is termed allopatric speciation. The physical separation could be the result of continental drift, uplift or subsidence of landmasses, glaciers, flooding, or radical dispersal of population members. Dispersal of a few fertile population members to an isolated island is a good example. The resulting gene pool, although small, is immediately isolated. For example, if the fruiting inflorescence of a common grass were transported on the struts of an airplane to a small, isolated island, any plants resulting from germination of those seeds would represent a new population, now isolated from the parent population on the distant landmass where the seeds originated. A drastic change in gene frequency could, and likely would, result; a gene present in one out of one thousand in the original population might, in the new, isolated location and within the smaller gene pool, increase in frequency from 0.05 percent to 25 percent if present in one of two plants forming the invading population. This sudden change in frequency is referred to as the founder’s effect. When such a population is introduced into a new environment, it may rapidly change and give rise to a number of additional new species. The latter process is known as adaptive radiation and is credited for the assemblage of unique species often found in isolated areas.

Sympatric Speciation

It is possible for segments of a parent gene pool to diverge without spatial separation, ultimately becoming reproductively isolated. This process is termed sympatric speciation. Examples of situations that may lead to sympatric speciation include the disruption of pollination that could result from different flowering times among members of a population. Such differences could be caused by variations in soil, moisture, or exposure. One example of this process is associated with the genus Achillea, or milfoil. In the late 1940s, investigators separated clumps of two species of milfoil to produce genetically identical clones that were subsequently transplanted to different elevations in the Sierra Nevada. Plants grown during this process exhibited a wide range of morphological variation. Not only did they look different; they flowered at different times. Thus, even genetically identical individuals of one species can be reproductively isolated if the timing of flowering precludes visitation by a common pollinator.

Polyploidy

Genetic variation can occur rapidly through an increase in chromosome number. Although the recombination of genes brought about by sexual reproduction provides the greatest amount of the observable variation in a population, abrupt and large-scale change is also associated with a process called polyploidy. Through polyploidy, a plant can have its entire set of chromosomes multiplied. If the increase in chromosome number is brought about when chromosomes fail to separate after they duplicate during meiosis, an individual’s number of chromosomes may double. This process is known as autopolyploidy. An increase in chromosome number associated with hybridization resulting from the combination of two separate sets of chromosomes is termed allopolyploidy. Although typically sterile, an allopolyploid may duplicate its combined set of chromosomes, resulting in a fertile autoallopolyploid.

Although polyploidy occurs naturally in many plants, the results so frequently are associated with desirable changes in the bloom of ornamentals that polyploidy is often deliberately induced by chemical treatment. It is estimated that one half or more of all flowering plant species may have arisen through some form of polyploidy.

Hybridization

Offspring produced from the interbreeding of related species (different species under the same genus), hybridization, may be either sterile or fertile. If the offspring are sterile, the genetic pools of parental species are not altered, because genes are not able to flow between the two. If the offspring are fertile, as are hybrids of crosses between North American and European species of sycamores, interbreeding among fertile hybrids and parent populations can provide a bridge for the merging of genetic information. With time, the flow of such genetic material can lead to an increase in variation, a decrease in interspecific differences, and an interesting taxonomic problem as to when the two parent species should be reclassified as one. Geographic isolation maintains the genetic integrity of these parental species.

Recombination Speciation

Although the original source of variation is permanent change in DNA (mutation), recombination of genetic material through sexual reproduction accounts for the vast majority of variation between generations. The result of this variation, acted upon by natural selection, leads to changes in the frequencies of genes within a population. The latter is often stated as a definition of evolution. Rapid selection of recombinants has been proposed as the method of speciation by which the anomalous sunflower, Helianthus anomalus, was produced from hybrids of two other species of sunflower, Helianthus annulus and Helianthus petiolaris. Studies, in fact, have duplicated the proposed process. After several generations of natural selective pressure, experimentally produced hybrids of Helianthus annulus and Helianthus petiolaris were demonstrated to be the genetic equivalents of naturally occurring Helianthus anomalus.

Sterile Hybrids

Plant hybrids often are sterile because newly combined hereditary material is so different that chromosomes will not pair during meiosis. Viable gametes, therefore, are not produced. This apparent genetic blind alley does not always translate into a lack of evolutionary success, however. Many sterile hybrids demonstrate an ability to reproduce apomictically, meaning they reproduce vegetatively rather than sexually. Species of blackberry, aspen, and many grasses reproduce by apomictic methods. Some, like dandelion, may have populations that are fertile and populations of infertile hybrids that produce seed with embryos that were produced asexually. Apomixis obviously does not promote variation, but it may permit the expansion of populations where there is little chance for cross-pollination, as in areas of high disturbance or environmental stress.

Hybrid animals are rare; most animals will not mate outside of their species. Like plant hybrids, animal hybrids are often sterile because of their different numbers of chromosomes. A common example is the mule, a cross between a female horse and a male donkey. Mules typically cannot reproduce because they have sixty-three chromosomes, a different number and structure than the donkey's sixty-two chromosomes and the horse's sixty-four, thus making it difficult during mating for chromosomes to pair up properly and create embryos.

Reproductive Isolation

Once a gene pool has separated and diverged into populations identifiable as separate species by genetic or morphological traits, the resulting segments of the pool must remain separated. As long as they are separate and distinct, populations can be labeled as separate species.

Barriers that prevent the flow of hereditary material may be classed as prezygotic or postzygotic. Prezygotic mechanisms prevent successful fertilization and include geographic isolation, temporal isolation (flowering at different times), mechanical isolation (flowers structurally different), behavioral isolation (incompatible courtship ritual), and incompatible gametes. The red-legged frog (Rana aurora) and the bullfrog (Rana catesbiana) are an example of environmental and spatial isolation—when the geographic ranges of two species overlap but their habitats or breeding conditions differ. These two populations are incompatible because the bullfrog breeds in permanent pond water whereas the red-legged frog breeds in fast-moving streams. Postzygotic mechanisms prevent the production of fertile adults and include hybrid inviability (offspring that do not live to sexual maturity), hybrid sterility, and offspring of hybrids that are weak.

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